P384: LOW CD19-MFI IN BALL CELLS BEFORE CAR T INFUSION PREDICTED A GOOD RESPONSE AND LONGER MRD-NEGATIVE MAINTENANCE TIME
Authors: Ping Wang, Xiaojuan Deng, Yun Zhang, Yi LI, Yuxuan Du, Xiangui Peng, Cheng Zhang, Yimei Feng, XI Zhang
Journal: HemaSphere (2023)

Background: Target protein positive of Leukemic cells is the key to the success of CAR T cell therapy. The expression of target proteins in tumor cells is usually evaluated by flow cytometry, and the percentage of CD antigen greater than 20 % is considered as the cutoff value of positive target in flow cytometry. However, tumor cell target positive CD antigen showed different fluorescence abundance (weak expression, homogeneous concentrated expression, or heterogeneous continuous expression). The Mean Fluorescence Intensity (MdFI) of the target CD is not strictly required for evaluating the response to CAR-T therapy. Whether the difference in CD fluorescence abundance of the target affects the response to CART treatment has not been reported. Aims: To investigate the use of mean fluorescence intensity (MFI) of B-ALL leukemia cells to predict the response rate and duration of efficacy after anti-CD19-CART cell therapy. Methods: Patients receiving the same CD19-CAR product were retrospectively analyzed for treatment response and follow-up results. Firstly, the flow standardized monitoring protocol of minimal residual disease (MRD) in r/r B-ALL patients and the normalized standard method of CD19-MFI flow cytometry were established. Secondly, two methods, CD19-MFI (CD19-MdFIR) and tumor load (FCM-MRD+), were used to compare the response rate and efficacy maintenance time of CART treatment. Results: 23 r/r B-ALL patients were injected with a dose of 0.5-1.5 × 105 CAR T cells/kg from January 2018 to June 2020. The mean value of CD19-MFI (CD19-MdFIR) of mature B cells in normal bone marrow (n = 15) and in the r/r BALL patients(n=23) were 545.54 ± 18.55(2.71 ± 0.09) and 545.22 ± 42.25(2.73 ± 0.21) respectively by flow cytometry (P > 0.05). Therefore, in this study, CD19-MFI = 545 and CD19-MdFIR = 2.73 were used as the criteria to determine the high and low fluorescence boundaries of leukemia cells. For CAR response,The median obtained MRD negative time of Low CD19MdFIR group(n = 9) and High CD19MdFIR group(n=14) were 14 vs 28 days (P = 0.0333). During follow-up,The MRD negative maintenance time of Low CD19MdFIR group was not defined, and the median MRD negative maintenance time of High CD19MdFIR group was 7.5 months (95%CI 0.075 to 0.912) (P = 0.0353). According to the tumor load group, the median time to MRD-negative in high disease load group (bone marrow Blast cells ≥ 5%)(n=12) and low disease load group (FCM-MRD+ LAIP cells < 5%)(n=11) were 28 days (P = 0.0940) in CAR response. At follow-up, the median MRD-negative maintenance time was 7 months in the high-burden group (95% CI 1.51 to 27.45). The duration of MRD negative maintenance in the low disease load group was not defined (95% CI 0.036 to 0.663), and there was a significant difference between the two groups (P = 0.0119). In the tumor load subgroup, the median time of MRD negative maintenance in the Low tumor load + Low CD19MdFIR group (n = 5) was not defined (95%CI 0.01258 to 0.6818). The median MRD negative maintenance time of low tumor load + High CD19MdFIR group (n = 6) was 9 months (95%CI 1.467 to 79.48) (P = 0.0195). In the High tumor load group, the MRD negative maintenance time of Low CD19MdFIR (n = 4) and high CD19MdFIR (n = 8) was about 25.00 months vs 4.00 months (P = 0.0783). Conclusion: Low CD19-MFI and CD19-MdFIR in B-ALL cells before CAR T infusion can be used as markers for good response and longer MRD-negative maintenance time. And the predictive efficacy of CD19 MFI is no less effective than that of tumor load assessment. Keywords: CAR-T, MRD, CD19, Acute lymphoblastic leukemia